Cloud systems which are accessed by clients by causing a data center to operate a server have increased more and more. The data centers of the cloud systems are often located geographically far away or abroad. Accordingly, in the cloud systems, there is a tendency that a round trip time (RTT) between a client terminal and a server becomes more significant.
In an environment with a large RTT, when a transmission control protocol (TCP) communication is carried out, dropping of data causes reduction of throughput. This is caused by TCP characteristics that a long time is taken until the throughput is recovered after the throughput has once been lowered due to occurring of congestion. In general, the TCP adjusts the throughput by changing the size of a congestion window (cwin) on the basis of the congestion.
The methods of controlling a congestion window in the TCP are broadly classified into two types. One type is a congestion control algorithm of enlarging the congestion window depending on the number of ACKs received from a destination and an example thereof is a scalable algorithm. The other type is a congestion control algorithm of increasing the congestion window depending on the elapsed time from the generation of congestion and an example thereof is a cubic algorithm.
Recently, as a countermeasure for improvement of the throughput in an environment with a larger RTT, devices such as a wide area network (WAN) speed-up device have been used. The WAN accelerator is a device improving a decrease in average throughput of the TCP by replacing the TCP of an end device with a protocol having a higher line utilization rate and performing a communication. The WAN accelerator may be a dedicated appliance or may be a virtual appliance which is virtually executed on a computer or the like.
For example, in a cloud system, a WAN accelerator A and a WAN accelerator B are arranged with a network such as a WAN line connecting a server and a terminal interposed therebetween. The WAN accelerator A temporarily terminates a TCP session accepted from the terminal. Then, the WAN accelerator A replaces data received through the TCP session with speeded-up protocol (hereinafter, also referred to as a high-speed protocol) data and transmits and receives the replaced data to and from the opponent WAN accelerator B. Thereafter, the WAN accelerator B restores the data transmitted to and received from the opponent WAN accelerator A through the high-speed protocol to the normal TCP data and transmits the restored normal TCP data to the server.
A protocol obtained by improving a user datagram protocol (UDP) to add a retransmission control function or a congestion control function thereto or a protocol obtained by more efficiently improving the congestion control function of the existing TCP is used as the high-speed protocol. For example, a UDT (UDP-based data transfer) protocol is known as the UDP based protocol. A CUBIC protocol or a Scalable protocol is known as the improved TCP. Particularly, the Scalable TCP is a protocol dedicated to a recent WAN line with a large RTT.    Patent Document 1: Japanese Laid-open Patent Publication No. 2010-74279
However, in the related art, there is a problem in that it is difficult to improve the latency for communications of transactions requiring a quick response.
Specifically, the WAN accelerator according to the related art uses a high-speed protocol with the maximum average throughput for communications in which a large amount of data are transmitted like bulk transfer and ACKs are frequently returned.
For example, in the bulk transfer, expected performance can be achieved by using the Scalable TCP more than the Cubic TCP, because the enlargement of a congestion window is more aggressive and the average window size is larger. Since the Cubic TCP enlarges the congestion window with the elapse of time after the congestion window has been reduced due to the occurrence of congestion, much time is taken until the congestion window is restored to the original state. That is, much time is taken until the average throughput reaches a desired value.
From this point of view, the WAN accelerator according to the related art often uses the Scalable TCP as a high-speed protocol with the maximum average throughput. However, in burst transmission like transactions, since the number of packets is small and the number of ACKs is also accordingly small, it is difficult to achieve desired performance through the use of the Scalable TCP. That is, since the WAN accelerator according to the related art often uses the Scalable TCP, it is difficult to say that the latency is improved for transaction communications requiring a quick response.